THE MORPHOLOGY AND PHYSIOLOGY OF THE SALAMANDER THYROID GLAND

III. THE RELATION OF THE NUMBER OF FOLLICLES TO DEVELOPMENT AND GROWTH OF THE THYROID IN Ambystoma maculatum

¹ From the University of Maryland Medical School and the Marine Biolcgical Laboratory, Woods Hole, Mass.

1. In the species Ambystoma maculatum, as contrasted to the species A. opacum, the number of follicles does not show, as a rule, the characteristic changes related to metamorphosis and consisting in the formation, by fusion of follicles, of a large, tube-like or bag-like follicle filled with colloid. Nevertheless a large, bag-like follicle making the largest part of the entire organ was found in one animal among 120.

2. In the species A. maculatum the average number of follicles iS 39.7.

3. The number of follicles in each thyroid remains constant during the entire life of the individual.

4. The growth of the thyroid is effected entirely by an increase in the size of the follicles.

5. The primary follicles are simple spherical vesicles.

6. As the follicles grow in size the surface becomes markedly irregular. The large follicles possess diverticula of varying number, size and shape or may be composed of two or more equally large and diverticulated component follicles.

7. Separation of the bud-like diverticula from the mother follicles or cleavage of the larger follicles into smaller follicles does not occur.

8. The bud-like diverticula and composite follicles are the result of fusion between adjacent follicles.

9. The source for the continuous formation of new follicles are probably small masses of epithelial cells found in many thyroids.

10. Owing to the fusion of the primary follicles, which goes on at the same rate as the formation of new follicles, the number of follicles remains constant.

11. In adult, sex-mature animals primary follicles cease to develop; those present do not fuse with larger follicles, but themselves begin to enlarge in size.